.\" Copyright (c) 2010 Alexander Motin <mav@FreeBSD.org>
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.Dd December 15, 2022
.Dt ATRTC 4
.Os
.Sh NAME
.Nm atrtc
.Nd AT Real-Time Clock (RTC) driver
.Sh SYNOPSIS
This driver is a mandatory part of i386/amd64 kernels.
.Pp
The following tunables are settable from the
.Xr loader 8 :
.Bl -ohang
.It Va hint.atrtc. Ns Ar X Ns Va .clock
controls event timers functionality support.
Setting to 0, disables it.
Default value is 1.
.It Va hw.atrtc.enabled
Forces enabling or disabling of the device(s).
Setting to 0 disables it, setting to 1 enables it, bypassing any platform
configuration hints.
Default value is -1 (autodetect).
.El
.Sh DESCRIPTION
This driver uses RTC hardware to supply kernel with time-of-day clock
with 1 second resolution and one event timer.
This hardware uses base frequency of 32768Hz for advancing time-of-day clock
and generating periodic interrupts.
Interrupts could be generated with fixed number of frequencies, from 2Hz to
8192Hz, obtained by dividing base frequency by one of supported power-of-2
divisors.
.Pp
Event timer provided by the driver is irrelevant to CPU power states.
.Sh SEE ALSO
.Xr apic 4 ,
.Xr attimer 4 ,
.Xr eventtimers 4 ,
.Xr hpet 4
